An addition condensation product between urea and an aqueous formaldehyde solution, which is mainly composed of a methylene compound, is widely used as a slow-releasing nitrogenous fertilizer, a molding material for urea resins, and the like, and methylol urea solution is an intermediate product of the addition condensation reaction between urea and aqueous formaldehyde solution.
The addition reaction between urea (either solid urea or an aqueous urea solution) and an aqueous formaldehyde solution is usually carried out by using urea and formaldehyde at a molar ratio of from 1:1 to 1:3 for the production of slow-releasing nitrogenous fertilizers or from 1:0.4 to 1:0.7 for the production of urea resins in the presence of an alkali agent with an initial pH value of the reaction system of from 7 to 10. The reaction is usually effected at a temperature of from 30.degree. C. to 90.degree. C. for a period of from several tens minutes to serveral hours. The resulting methylol urea solution comprising mainly of monomethylol urea and dimethylol urea is then rendered acidic and subjected to dehydration condensation reaction to obtain a condensation product comprising mainly of a methylene compound.
When substrate concentrations in the reaction system are low, for example, when an aqueous urea solution at a low concentration and an aqueous formaldehyde solution is used, the resulting methylol urea solution naturally has a low concentration. In the production of, for example, urea-formaldehyde condensate nitrogenous fertilizers, use of such a low-concentrated methylol urea solution attains a low reaction rate in the dehydration condensation and also requires filtration to separate a condensate and recycling of the mother liquor. In addition, since the thus separated condensate has a water content of about 70% by weight, it should be dried for an extended period of time, which entails a high cost.
On the other hand, when solid urea or a urea solution in a high concentration and aqueous formaldehyde solution are used, since the resulting methylol urea solution has a high concentration, the condensation product preferably obtained therefrom only requires removal of a water content of about 40% by weight by drying without involving complicated operations, such as filtration.
The addition reaction, when conducted under an alkaline condition as described above, is not substantially accompanied by condensation reaction so that monomethylol urea can be produced in high yields. As a result, for the production of slow-releasing nitrogenous fertilizers, the subsequent condensation is easy to control, and the resulting urea-formaldehyde condensate exhibits excellent slow-releasing properties as fertilizer. For the production of urea resins, such high methylol urea yields are advantageous to provide molding materials having satisfactory plasticity.
However, use of a commonly employed alkali agent, such as sodium hydroxide, ammonia, trisodium phosphate, etc., for the pH adjustment in the addition reaction cannot avoid pH reduction of the methylol urea solution with the passage of time during and after the reaction, e.g., during preservation. Such pH reduction is particularly conspicuous in the methylol urea solution prepared from solid urea or a high-concentrated urea solution and an aqueous formaldehyde solution. It is believed that the pH reduction is attributed to air-oxidation of active formaldehyde released from methylol ureas to form formic acid, or to Cannizzaro reaction. If the pH value is lowered to 7.0 or less, dehydration condensation commences to thereby cause white turbidity of the methylol urea liquid due to denaturation. Further progress of the condensation reaction results in precipitation of methylene compounds in the methylol urea solution, which is extremely unfavorable on handling.
Denaturation of the methylol urea solution due to pH reduction might be suppressed by occasional addition of an alkali agent in accordance with the pH reduction or initial addition of a large quantity of an alkali agent. However, the former method needs a complicated procedure, and the latter method unavoidably increases the pH value in the initial stage of the reaction. In particular, if the pH value is 11 or more, the production rate of dimethylol urea becomes so high that dimethylol urea is readily precipitated. Moreover, such a high pH value not only requires a large amount of an acid but also makes pH control difficult for rendering the methylol urea solution acidic to commence dehydration condensation.
The methylol urea solution suffering from denaturation due to pH reduction is difficult to handle, and also a urea-formaldehyde condensate obtained therefrom is inferior in slow-releasing properties as a fertilizer.
In general, effectiveness of nitrogenous fertilizers in chemical analysis is evaluated by quantitatively analyzing a total nitrogen (TN), a cold water-soluble nitrogen (WSN), a cold water-insoluble nitrogen (WIN), a hot buffer-insoluble nitrogen (HWIN), in accordance with the A.O.A.C. method (Association of Official Analytical Chemists) and calculating an AI value (i.e., aa ratio of a cold water-insoluble and hot buffer-soluble nitrogen to a cold water-insoluble nitrogen) through the following formula. ##EQU1##
Among urea-formaldehyde condensates, methylene urea having a low degree of condensation is generally soluble in water or a hot buffer solution and is effectively absorbed into plants as a nitrogen source, but the water-soluble methylene urea shows a relatively high rate of decomposition into inorganic nitrogen (mineralization) and is not, therefore, suitable as a slow-releasing fertilizer. On the other hand, highly condensed methylene urea that is insoluble even in a hot buffer solution has an extremely low rate of decomposition into inorganic nitrogen after fertilization and is hardly utilized by plants.
Accordingly, it has been desired that highly effective and slow-releasing nitrogenous fertilizers should have a low WSN content, a high WIN content, and a low HWIN content, i.e., a high AI value with a high WIN content.
However, when methylol urea solution obtained by conventional processes are subjected to dehydration condensation reaction under an acidic condition to prepare urea-formaldehyde condensate nitrogenous fertilizers, there is a general tendency that an increase of a WIN content is attended by an increase of an HWIN content, thus abruptly decreasing the AI value. That is, it has been very difficult to obtain slow-releasing urea-formaldehyde fertilizers having a high AI value as well as a high WIN content.